The use of diagnostic assays is very well known for the diagnosis, treatment and management of many diseases. In that regard, different types of diagnostic assays have been developed to simplify the detection of various analytes in clinical samples such as blood, serum, plasma, urine, saliva, tissue biopsies, stool, sputum, skin or throat swabs and tissue samples or processed tissue samples. These assays are frequently expected to provide a fast and reliable result, while being easy to use and inexpensive to manufacture.
One common type of disposable assay device includes a sample addition zone or area for receiving the liquid sample, at least one reagent zone (also known as a conjugate zone), a reaction zone (also known as a detection zone), and optionally an absorbing zone. These zones can be arranged in order along a fluid passage or channel. These assay devices, commonly known as lateral test strips, can employ a porous material, e.g., nitrocellulose, defining a path for fluid capable of supporting capillary flow. Examples include those devices shown in U.S. Pat. Nos. 5,559,041, 5,714,389, 5,120,643, and 6,228,660, all of which are incorporated herein by reference in their entireties.
The sample addition zone of these assay devices frequently includes a porous material, capable of absorbing the liquid sample, and, when separation of blood cells is required, also effective to trap the red blood cells. Examples of such materials are polymeric membrane filters or fibrous materials, such as paper, fleece, or tissue, comprising e.g., cellulose, wool, glass fiber, asbestos, synthetic fibers, polymers, or mixtures of the same.
Another type of lateral-flow assay device is defined by a non-porous substrate having a plurality of upwardly extending microposts (also referred to as “micropillars” or “projections”). The microposts are defined dimensionally and in terms of their spacing to produce capillary flow when a liquid is introduced. Examples of such devices are disclosed in U.S. Pat. No. 8,025,854B2, WO 2003/103835, WO 2005/089082, WO 2005/118139 and WO 2006/137785, all of which are incorporated by reference herein in their entireties.
A known non-porous assay device of the above type is shown in FIG. 1. The lateral-flow assay device 1 has at least one sample addition zone 2 configured to receive a sample 101, graphically represented using a teardrop shape. The sample 101 can include, e.g., a bodily fluid or other fluid to be tested for an analyte. The lateral-flow assay device 1 also includes a reagent zone 3, at least one detection zone 4, and at least one wicking zone 5, each disposed on a common substrate 9. The zones 2, 3, 4, 5 are aligned along a defined fluid flow path 64 by which the sample 101 or a portion thereof flows from the sample addition zone 2 to the wicking zone 5 under the influence of capillary pressure provided between ones of a -plurality of microposts 7. Capture elements, such as antibodies, can be supported in the detection zone 4, these elements being capable of binding to an analyte of interest, the capture elements being deposited on the device, e.g., by coating. The term “element” is not limited to atoms, i.e., chemical elements of the periodic table, but can also refer to molecules, e.g., of ionically or covalently-bonded atoms, or other chemical compounds or biological substances. In addition, a labeled conjugate material, also capable of participating in reactions that will enable determination of the concentration of the analyte, is separately deposited on the device in the reagent zone 3, wherein the conjugate material carries a label for detection in the detection zone 4 of the lateral-flow assay device 1.
The conjugate material is gradually dissolved as the sample 101 flows through the reagent zone 3, forming a conjugate plume of dissolved labeled conjugate material and sample 101 that flows downstream along the defined fluid flow path 64 of the lateral-flow assay device 1 to the detection zone 4. As the conjugate plume flows into the detection zone 4, the conjugated material will be captured by the capture elements such as via a complex of conjugated material and analyte (e.g., as in a “sandwich” assay) or directly (e.g., as in a “competitive” assay). Unbound dissolved conjugate material will be swept past the detection zone 4 and into the wicking zone 5.
An instrument such as that disclosed in U.S. 2006/0289787A1, U.S. 2007/0231883A1, U.S. Pat. Nos. 7,416,700 and 6,139,800, all incorporated by reference in their entireties herein, is configured to detect the bound conjugated material in the detection zone 4. Common labels include fluorescent dyes that can be detected by instruments which excite the fluorescent dyes and incorporate a detector capable of detecting the resulting fluorescence. In the foregoing devices and in the conduction of assays, the resulting level of signal in the detection zone is read using a suitable detection instrument after the conjugate material has been dissolved and the sample 101 and unbound conjugate material have reached and subsequently filled the wicking zone 5 of the lateral-flow assay device 1.
In a typical point of care (POC) lateral flow assay format, it is desirable to remove unbound conjugate materials to lower background signal and improve assay accuracy. In some assays, fluid of the sample 101 continues to flow through the detection zone 4 after all the dissolved conjugate passes the detection zone 4. In this way, the flowing sample 101 removes unbound conjugate materials. However, endogenous interferents may be present in the sample 101 that may interfere with assay results (e.g., hemoglobin, bilirubin of a particular patient). For these assays, wash fluid separate from the sample 101 can be applied to remove the interferent from the detection zone 4 or other parts of the detection channel. Moreover, some assays involve pre-mixing the conjugate material with the sample 101 prior to addition of the mix to the sample addition zone 2 to obtain a longer incubation time. For these types of assays, since the sample 101 is mixed with the conjugate, a wash fluid is applied to remove unbound conjugate from the detection zone 4. In these and other embodiments, wash fluid can be formatted or designed to provide an acceptable wash. Accordingly, adding wash fluid is necessary for some selected assays in, e.g., a POC lateral flow format.
However, adding wash reagent is a challenge in various prior lateral-flow assay devices. The wash fluid is to flow in the gaps between pillars (or in the pores of a porous structure, such as cellulose acetate). However, since flow resistance in gaps or pores is much larger than outside of the pillar matrix (or porous) structure, wash fluid cannot be “pushed” into the fluid flow path 64 (the pillar matrix) to accomplish the wash. Wash fluid has to be “pulled” into the gaps between pillars or pores of a porous material by the capillary pressure. There is therefore a need for assay devices and ways of using assay devices that are more compatible and usable with various wash fluids.